Longitudinal optical (LO) and LO-like phonon resonant mid-infrared radiation emission and absorption by surface micro-structures on III-nitrides

Longitudinal optical-like phonon resonant mid-infrared emission (LORE) and absorption peaks of micro-surface line and space structures of metallic material plates on GaN wafers are observed at 500 – 630 K. The emission peak energy is located in a range of 670 – 730 cm⁻¹ for GaN, which is lower than the LO-phonon energy of approximately 730 cm⁻¹ at 630 K. This feature is contrasted with that of the LO-phonon resonant emission from the Au-GaAs microstructures resonating with the LO phonon. The emission mechanism is clarified using a structure of n++-GaN with an electron density of 1×10²⁰ cm⁻³ as a metallic material. These emission lines have another notable feature, i.e., the observed peak energies are independent of the polar emission angle, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The present results show that each peak energy in the emission spectrum is positioned at the zero-point of the real part of the dielectric function comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. This type of emission is also obtained for the structures of AlxGa1-xN films. These results suggest the feasibility of high-efficiency emission in a range of 670 – 900 cm⁻¹ when the devices are positioned in a cavity with high reflectivity of blackbody-like radiation.